1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More particularly, the invention relates to a pressure medium fluid circuit to be adapted for the active suspension system, which achieves optimal fluid discharge performance.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner to the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as a proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit to be controlled the valve position. The pressure control valve is controlled the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and whereby controlling suspension characteristics.
On the other hand, European Patents 0 283 004, 0 285 153 and 0 284 053 disclose technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
In one of the typical construction of the hydraulic circuit includes a pressure source unit which comprises a fluid pump drivingly associated with an automotive internal combustion engine so as to be driven by the engine output torque. The fluid pump is generally rated to produce rated pressure which is selected in view of the required line pressure in a supply line for supplying the pressurized fluid to the working chamber, at the minimum revolution speed of the engine so that the working fluid pressure to be supplied to the working chamber of the hydraulic cylinder can be satisfactorily high at any engine driving range. As will be appreciated, the output pressure of the fluid pump increases according increasing of the engine revolution speed. Therefore, at high engine revolution speed range, excessive pressure in excess of a predetermined maximum line pressure is relieved via a relief valve. Therefore, the engine output can be wasted to degrade engine driving performance as a power plant for the automotive vehicle and thus degrade fuel economy.
On the other hand, in the practical operation of the active suspension system, the fluid pressure in the working chamber in the hydraulic cylinder can be maintained at constant value for maintaining a desired vehicular height, at substantially low vehicle speed range or while the vehicle is not running. Despite this fact, the prior proposed hydraulic circuits for the actively controlled suspension systems supply the rated pressure of the fluid pump which should be higher than a minimum line pressure required for adjustment of the fluid pressure in the working chamber.
Improvement in the hydraulic circuit for the prior proposed active suspension system has been proposed in the co-pending U.S. patent application Ser. No. 331,602, filed on Mar. 31, 1989, which application has been commonly assigned to the common assignee to the present invention. The corresponding invention to the above-identified co-pending U.S. Patent Application has been published as Japanese Patent First (unexamined) Publication (Tokkai) Heisei 1-249509, published on Oct. 4, 1989. The prior invention has been directed to a hydraulic circuit for an actively controlled suspension system which employs first and second pressure relief valves disposed in a hydraulic pressure source circuit for relieving excessive pressure. The second pressure relief valve is provides a lower relief pressure than that of the first pressure relief valve. Means for selectively connecting and disconnecting the second pressure relief valve is disposed in the hydraulic pressure source circuit at an orientation upstream of the second pressure relief valve. The means is positioned at shut-off position to disconnect the second pressure relief valve when a vehicle traveling speed is higher than a predetermined speed. The means is responsive to the vehicle speed lower than the predetermined speed for establishing connection between a pressurized fluid source to the second relief valve for relieving the pressure at lower level than that established when the vehicle speed is higher than the predetermined speed.
Furthermore, the prior proposed invention includes a pilot pressure operated one-way check valve in a drain line for regulating line pressure to be supplied to a pressure control valve which adjusts fluid pressure in a working chamber in a hydraulic cylinder disposed between a vehicle body and a suspension member rotatably supporting a road wheel, by draining excessive line pressure. Similar hydraulic circuit constructions have also been disclosed in European Patent First Publications Nos. 0 318 721, 0 318 932, for example.
In the prior proposed active suspension systems, the fluid pressure is supplied from a fluid pump which is driven by an automotive internal combustion engine. As can be appreciated, such type of the fluid pump is variable of discharge rate depending upon the engine revolution speed. In order to prevent pulsation in the line pressure, a pressure regulating means is provided in the pressure supply network. On the other hand, when acceleration of stroke in relative bounding and rebounding motion of the vehicle body and the suspension member, increased discharge rate of the fluid pump is required so as to compensate the relatively large amount of working fluid supplied and drained into and from a working cylinder. Japanese Patent First (unexamined) Publication (Tokkai) Showa 63-251313 has discloses the pressure supply system having capability of adjustment of the discharge rate of the fluid pump depending upon relatively large magnitude of bounding and/or rebounding acceleration.
The proposed system is particularly advantageous when the vehicle is in the parking, resting or not running state because of smaller discharge rate of the fluid pump for smaller power loss in the engine. On the other hand, the proposed system is so designed as to command increasing of the discharge rate in response to large magnitude of bounding and rebounding acceleration. However, the signal representative of bounding and rebounding magnitude does not necessarily correspond to the fluid flow rate to be consumed in the active suspension system. Therefore, the discharge rate of the fluid pump as controlled by the command generated in response to large magnitude of bounding and rebounding acceleration cannot precisely correspond to that required in the active suspension system. Therefore, the line pressure tends to become excessively high to lead wasting of the engine output power and excessively low to lead in sufficient suspension adjusting performance.